As disclosed in several previous applications including the above-mentioned U.S. Ser. No. 12/378,666 and U.S. Ser. No. 12/210,096, a spherical remote phosphor can have very uniform luminance, and thereby a uniform spherical intensity. Phosphor-LED light systems typically use blue LEDs and a yellowish phosphor, which combine to produce a white light. An aesthetic drawback of a large spherical remote phosphor in some cultures and contexts, however, is its strongly yellowish appearance when the lamp is unlit and no blue light is present. A further aesthetic drawback is that the shape of the remote phosphor lamp is usually substantially different from that of conventional light bulbs, with their sphere-on-a-threaded-stalk look. What is needed is an LED lamp with the same shape as a traditional incandescent bulb, but with adequate heat-removal capability for efficient use of LEDs and phosphors, especially when tasked to produce the same high luminosity as a 75-Watt incandescent bulb, at far lower power.
The prior art includes U.S. Pat. No. 7,479,662 to Soules, et al., which discloses a transparent sphere with a blue LED chip at its center and a phosphor coating on its surface. FIG. 4 in Soules shows an LED chip 312 mounted in the center of a molded sphere 318 which has a “phosphor layer . . . coated on the inside surface of the sphere.” Soules states that the LED “will radiate uniformly in all directions”. However, Soules does not provide details of the LED that will achieve that uniform spherical light distribution. Commonly available LEDs typically produce a hemispherical (or near hemispherical) Lambertian intensity pattern, which is well known to be quite non-uniform. There are also some LEDs with batwing or other non-uniform intensity patterns, but none with hemispheric uniformity. Instead, the hemispherical Lambertian output of a typical packaged LED or chip gives a non-uniform distribution of blue light onto the phosphor coating in a hemisphere (only half a sphere), resulting in non-uniform surface chrominance, with the highest color temperature above the chip and the lowest behind it.
In the embodiment shown in FIG. 4 of Soules, if LED chip 312 does not emit spherically (as is required by Soules) but is a hemispherical Lambertian source, then the upper hemisphere of the phosphor coated inner surface of the hollow ball will be directly lit by blue light striking it from the LED. The lower hemisphere of the phosphor coated surface cannot be lit directly but is lit only by the meager blue light reflected from the upper hemisphere.
Measurements done by the inventors and other researchers in the field of remote phosphor LED light sources (e.g. N. Narendran, Y. Gu, J. P. Freysinnier-Nova, Y. Zhu, “Extracting phosphor-scattered photons to improve white LED efficiency”, Phys. Stat. Sol. (a) 202 (6): R60-R62, Rapid Research Letters, 2005 Wiley-WVH, see FIG. 3) show that typically the percent of reflected blue light from a transmissive phosphor layer designed to produce white light is around 10 to 15%, largely independently of the density of the phosphor coating (see FIG. 3 of Narendran et al). That is to say, 85 to 90% of the blue light will either be converted or pass unconverted through the phosphor layer on the upper hemisphere. Approximately 40 to 50% of the converted yellow light from the upper hemisphere will be emitted inwardly (see FIG. 3 of Narendran et al) and travel toward the lower hemisphere. In order for the white light ultimately emitted from the bulb to be the same on both hemispheres (same intensity, color temperature, etc) the amounts of yellow and blue light (and their ratio) must match the upper hemisphere at all points on the sphere. This presumably is possible somehow, but with an LED at the center of the sphere it is not obvious how this can be accomplished as is described by Soules et al. There is an additional problem to be overcome as a consequence of the non-uniformity of the light striking the different vertically located zones of the phosphor, light radiated non-uniformly from the Lambertian emitting LED. The intensity from a Lambertian source varies as a function of the cosine of the angle away from normal the ray is emitted. The intensity of any Lambertian surface drops to zero when the ray is perpendicular to the normal, precisely because it is parallel to the plane of the source. Therefore, the system of FIG. 4 of Soules would be unable to achieve uniform white light using LEDs that have a Lambertian output. This presumably is why Soules states that his system operates with LEDs that produce “uniform” output.
Soules in his FIG. 2 shows a more practical embodiment of his invention, one with a hemispherical remote-phosphor cover. That overcomes the problem stated previously in the embodiment of his FIG. 4, as it eliminates the lower-hemispheric section. Soules does not, however, address the paramount issue of the Lambertian output of typical LEDs and presumably relies on the LED to somehow produce “uniform” light in all angular directions within the upper hemisphere.